For example, in a centrifugal compressor, due to excitation force of a high pressure gas acting on a rotor thereof, the rotor causes self-excited vibration, so that the compressor has to be stopped sometimes. In general, in a rotation machine, it is known that large vibration is generated at critical speed where a natural frequency of a rotor thereof and the rotation number of the rotor are matched.
A technique described in Patent Document 1 is a technique whose object is to enable a rotor of a rotation type fluid machine to be operated even in the vicinity of quaternary critical speed. In a uniaxial type centrifugal compressor, a magnetic bearing is installed in a shaft end of a rotor where magnitude of a quaternary vibration mode is large, and generated magnetic force is controlled in such a manner that vibration of the rotor is reduced at the time of passing through the quaternary critical speed.
Patent Document 2 describes a technique for monitoring a state of stability of a shaft vibration system and detecting a sign of abnormal vibration in advance with a configuration that magnetic bearings are provided in both ends of a rotation shaft in a uniaxial type centrifugal compressor. The rotor at the time of operation is oscillated via the magnetic bearings, and a shaft vibration response at the time is detected by a sensor. A transfer function is calculated from an oscillation command and a detected value of the sensor, and a forward rotation mode and a backward rotation mode are separated from the calculated transfer function. From the obtained sensitivity function, a peak value of the mode serving as a monitoring object is read, and by displaying a time change thereof, the stability of the shaft system is monitored.